The present invention relates to an optical transmission system. More particularly, the invention relates to a high reliability fiber optic mode scrambler utilizing a phase-only filter.
Guided optical transmission systems are well known systems for rapid and highly reliable transmission of information. The basic guided optical transmission system consists of a transmitter, a receiver, and a guided channel connecting the two. Typically, an optical signal is introduced to the guided channel by the transmitter, travels through the guided channel and is then detected by the receiver.
Furthermore, a typical guided optical transmission system will include multiple guided channels, such as optical fibers, which are connected with connector elements. Such fiber-to-fiber connections are needed for a variety of reasons. Several fibers must be spliced together for lengths of more than a few hundred kilometers because only limited continuous lengths of fiber are normally available from manufacturers. Also, moderate lengths of fiber are easier to install in most applications than are very long cables.
Transmitters in typical guided optical transmission systems are either light emitting diodes (LEDs) or laser diodes (lasers). An LED is a pn junction semiconductor that emits light when forward biased. LEDs are incoherent light sources. Typical LEDs do not emit great amounts of power and are relatively slow. On the other hand, lasers have significantly more power and can be operated much faster. Lasers are coherent light sources.
The guided channel in a typical guided optical transmission system is a step-index fiber consisting of a central core with a designated refractive index surrounded by a cladding with a lower designated refractive index. Step-index fibers have three common forms: a glass core, cladded with a glass having a slightly lower refractive index; a silica glass core, cladded with plastic; and a plastic core, cladded with another plastic. The core/cladding interface allows a properly oriented optical signal to propagate within the core of the fiber with nearly total internal reflection, that is, none of the signal leaks into the cladding. As long as the signal enters the core of the fiber at an angle less than the critical angle, nearly all of the signal will remain in the core of the fiber so that there is little loss in the optical signal.
The fiber in a guided optical transmission system can be a single mode fiber or a multimode fiber. The multimode fiber has a multitude of transmission modes in which an optical signal can travel while the single mode fiber has only one transmission mode for an optical signal. The advantage of a multimode fiber is that significantly more information can be transmitted through the multimode fiber. A single mode fiber, limited to a single mode of transmission, is able to transmit much less information than a multimode fiber.
Most applications in which optical transmission systems are used require the extremely rapid transmission of data. Often, speed considerations will dictate that a laser source is necessary, as is the use of a multimode fiber so that more information can be transmitted at the same time. In addition, as stated above, most practical applications will require multiple fiber lengths utilizing connectors. The combination of highly coherent laser sources with a multimode fiber utilizing connector elements presents several problems.
Modal noise is one difficulty in such guided optical systems. When highly coherent light sources such as laser diodes are used with multimode fibers, the coherent source excites very few modes in the fiber. However, the fiber modes then interfere with one another causing random variations in optic power. This random power variation is known as modal noise. With typical LED sources, mode interference is not a problem because the light is so incoherent that interference will not greatly affect the overall power detection. However, with highly coherent light sources such as lasers, the modal interference can be both additive and subtractive such that any one of the few modes transmitting the optical signal can have a very significant portion of the optical power concentrated in that one mode.
This concentration in a single mode becomes extremely significant in systems which utilize mode-selective loss elements such as connectors. These connectors between the fibers cause losses in the optical signal. It is very difficult to perfectly align the fibers, even with the use of connector elements. Even slight misalignment of the cores of the fibers will cause mode-selective loss. Elements such as connectors are therefore sometimes called mode-selective loss elements. Certain transmission modes are cut off or terminated by these elements. Thus, such connectors may cause a signal to lose its higher order modes. When the terminated modes are carrying a significant amount of the optical signal power, there will be signal error at the detector. Consequently, multimode fibers cannot be used in many high speed applications.
One approach to overcoming these problems has been to use mode scramblers with corrugated surfaces to microbend the fibers. Such corrugated mode scramblers physically bend the fiber such that the angle of reflection between the signal and the core/cladding interface will be altered as the signal passes through the portion of the fiber which is bent by the corrugated mode scrambler. In this way, the optical signal will be reflected into many more modes than the few in which the coherent laser source originally transmits the signal. Thus, the corrugated scrambler can approximate equilibrium power distribution in the fiber.
By approximating equilibrium power distribution the effect of mode-selective loss element such as connecters are greatly lessened. When there are only a few modes in which the optical signal is traveling, any modes cut off by the mode selective loss elements will greatly diminish the overall signal power and cause error in detection. When more modes are utilized, however, each mode terminated by the mode-selective loss elements has much less impact.
Despite the advantages of such corrugated mode scramblers, there are many limitations to these devices, and their usefulness is very limited. First, corrugated mode scramblers impose intolerable strain on the fiber. In order for the corrugated scrambler to be effective, significant strain must be put on the fiber. The corrugated scrambler alters the angle of reflection between the core/cladding interface by physically bending the fiber. This bending stretches one side of the fiber and places tension on the other. In most applications this strain is intolerable. Most fibers are comprised of glass or plastic. Any strain on these fibers increases the risk that they will break. Even slight bends in fiber can cause cracks. These cracks can affect the optical signal traveling through the fiber and will eventually lead to breakage of the fiber. A broken or cracked fiber will not properly transmit an optical signal.
Second, in order for the corrugated scrambler to effectively approximate equilibrium power distribution in the fiber, multiple bends in the fiber are necessary. The fiber must be subjected to a series of alternating bends. The package size needed to contain these plurality of bends is too large for many practical implementations. Consequently, the corrugated scrambler has limited practical usefulness.
The present invention is an optical transmission system which solves these and related problems in the prior art.